Project Summary Realizing the full potential of otoacoustic emissions (OAEs) as noninvasive probes of cochlear function requires understanding the physical and physiological mechanisms that generate and shape these sounds. Despite considerable recent progress identifying fundamental differences among OAE types, many aspects of OAE generation remain controversial and poorly understood. Over the next five years we propose to test models of OAE generation and to explore hypothesized interrelationships between OAEs and other measures of cochlear function, including auditory-nerve tuning and basilar- membrane motion. In Aim 1 we will determine the relationship between cochlear tuning and OAEs in individual ears by measuring the amplitude and phase of auditory-nerve tuning and stimulus-frequency otoacoustic emissions (SFOAEs) using the same stimuli. In addition, we will use the data both to determine the functional characteristics of cochlear wave propagation and amplification, including their dependence on characteristic frequency and intensity, and to test models of OAE generation throughout the cochlea. We will test our auditory-nerve based conclusions with mechanical measurements in both the low- and high-frequency regions of the cochlea. In Aim 2 we will extend our knowledge of OAE mechanisms to the apical half of the cochlea, where mounting evidence suggests the mechanics are very different from the base. Although the apical half of the cochlea is the most important region for human speech communication, relatively little is known about its mechanical or otoacoustic responses. In Aim 3 we will determine modes of reverse energy propagation in the cochlea by testing model predictions for standing-wave interference patterns measurable on the basilar membrane. In Aim 4 we will synthesize our knowledge of cochlear mechanics and OAE generation through continued development of the physics-based cochlear and OAE models responsible for the hypotheses and predictions tested in the other Aims. Completion of these four interconnected Aims will significantly enhance our understanding of OAE generation mechanisms and their relationship to cochlear mechanics and tuning throughout the cochlea. In addition to the basic science they address, these Aims are directly relevant to improving the power of OAE-based hearing diagnostics and other technological applications, including pre-processors for speech-recognition and cochlear implant systems that benefit from knowledge of human cochlear tuning.